FIG. 10 illustrates a circuit of a conventional electric power converting device for once converting alternating-current (AC) electric power to direct-current (DC) electric power and further converting it to AC electric power. In FIG. 10, to one end of an AC power source 1, a series circuit of semiconductor switching elements 10, 11 is connected through a reactor 40. To the semiconductor switching elements 10, 11, diodes 14, 15 are connected in inverse parallel, respectively. The semiconductor switching elements 10, 11 undergoes PWM (Pulse Width Modulation) so that they operate as a rectifier circuit together with the diodes 14, 15 to control the converting operation while storing energy in capacitors 30, 31 connected in series so that the voltage of the capacitors 30, 31 becomes a specified DC voltage.
Moreover, to the series circuit of the capacitors 30, 31, a series circuit of semiconductor switching elements 12, 13 are connected in parallel. To the semiconductor switching elements 12, 13, diodes 16, 17 are connected in inverse parallel, respectively. Here, by operating the semiconductor switching elements 12, 13 as an inverter by the PWM control, a stabilized arbitrary AC voltage is generated from a smoothed DC voltage. The AC voltage is supplied to a load 6. A capacitor 32, namely a filter capacitor, is connected across the AC power source 1. A reactor 41 and another capacitor 33 are connected to the input side of the load 6 to form an LC filter. A circuit similar to FIG. 10 is described in Power Electronics Guidebook,” Supplementary Volume of OHM Magazine, November 1999 issue, p85, OHM-sha.
FIG. 10 illustrates a so-called double converter arrangement, which once converts an AC power source voltage to a DC voltage before converting it to back an AC voltage. FIGS. 11A and 11B schematically illustrate the operating principle of the circuit of FIG. 10. In the circuit of FIG. 10, a converter, made up of the semiconductor switching elements 10, 11 and the diodes 14, 15 on the AC power source side, operates as a rectifier circuit. Therefore, the rectifier circuit, as shown in FIGS. 11A and 11B, can be regarded as a parallel current source 5 through which total energy required for the load 6 passes. Moreover, a converter, made up of the semiconductor switching elements 12, 13 and the diodes 16, 17 on the load side of FIG. 10, operates as a so-called inverter to supply a specified voltage to the load 6. Therefore, the converter, as shown in FIGS. 11A and 11B, can be regarded as a parallel voltage source 3 through which the whole energy required for the load 6 passes. Here, the capacitors 30, 31 of FIG. 10 are connected to the output side of the rectifier circuit (i.e., the input side of the inverter), and function as a power source of the inverter. In an electric power converting device of a double converter system as shown in FIG. 10, all of the energy supplied to the load 6 passes through both the converters on the AC power source side and on the load side. In this configuration, the loss generated by each of the converters becomes large. Thus, the problem of reducing conversion efficiency increases the running cost.
Furthermore, both the rectifier circuit and the inverter operate as half bridges to cause a voltage of about twice the AC power source voltage to be applied to the switching elements. Thus, the applied elements require a high breakdown voltage. This further increases the cost.
Accordingly, there remains a need for an electric power converting device that 1) runs more efficiently to cut down the running cost, 2) supply a constant voltage to a load while suppressing voltage variation in an AC power source, 3) reduces the breakdown voltage of circuit elements, such as semiconductor switching elements, to further reduce the cost, 4) operates the first and second switching element series circuits when there is a failure of the AC power source to thereby make it possible to continuously supply energy to a load, and 5) can comply with a plurality kinds of AC power source voltages without changing circuit arrangement or components. The present invention addresses these needs.